Forming a perforate membrane by laser drilling and a subsequent electro-polishing step

ABSTRACT

A method of forming a perforate membrane ( 1 ) is disclosed for use in a liquid transport device. The membrane has at least plural nozzles ( 10 ) formed therethrough. Each of those nozzles has a throat portion ( 12 ) opening at opposite end through opposite surface ( 2 ′) of the perforate membrane and a smoothly curved outwardly diverging portion ( 11 ) extending from the first end of the throat portion to the first surface ( 2 ) of the perforate membrane. Laser energy is applied selectively to the first surface ( 2 ) of the membrane in the form of a pulsed, focused beam to form the nozzles ( 10 ) and thereafter the first surface ( 2 ) of the membrane and the surface of the diverging portion ( 11 ) of the nozzles ( 10 ) are electro-polished to remove surface imperfections. The electro-polishing is controlled so as to remove material from the surface of the diverging portion ( 11 ) of the nozzles to a depth less than the length of the throat portion ( 12 ).

The present invention relates to nozzles, and more particularly, nozzlesIn perforate membranes for use in fluid transfer devices. Such devicesinclude aerosol generators, fluid pumps, and filter membranes. In suchdevices, fluid is transferred through the nozzles in the membrane.However in each case, the membrane provides certain properties that canbe related to the geometry of the nozzles contained therein.

Perforate membranes are known in aerosol creating devices, where bulkliquid is transferred from the source side of the membrane, through thenozzles, and disrupted so as to create droplets at the opposite(emergent) side of the membrane. Various devices are disclosed inWO-A-95/15822, U.S. Pat. No. 5,518,179, U.S. Pat. No. 5,152,456 and U.S.Pat. No. 4,533,082, for creating aerosol droplets using a vibratingperforate membrane. These devices use some differing methods fortransferring the liquid through the nozzles, and to create droplets atopposite side of the membrane.

In U.S. Pat. No. 5,152,456, bulk liquid is brought at ambient or nearambient hydrostatic pressure to a surface of the membrane (liquid-side),at which the cross-sectional area of the nozzles intersecting thissurface is larger than the cross-sectional area of the same nozzlesintersecting the opposite surface (air-side). For a stationary membrane,a liquid that wets the membrane material, and ambient pressureconditions, the fluid meniscus within the nozzle travels through thenozzle by capillary action to stabilise its position at the air-side ofthe nozzle (where the surface area of the meniscus is at a minimum).Thus the nozzle becomes liquid-filled. During operation, a periodicbending-mode vibration is generated in the perforate membrane, whichharmonically displaces the membrane towards and away from the bulkliquid, resulting in a varying hydraulic pressure in the liquid near tothe liquid-side of the membrane. Such pressure causes liquid to transferthrough the nozzles in a periodic manner. The transfer is from theliquid side to the air-side as the pressure increases above the ambienthydrostatic pressure. When the momentum with which the liquid istransferred through a nozzle and towards the air side of the perforatemembrane is sufficiently large, part of the liquid so transferreddetaches from the bulk liquid and, under the influence of its surfacetension, it then forms a droplet, which travels away from the air-sideof the membrane. This can occur simultaneously for many or all nozzleswithin the membrane.

The droplet diameter ejected from such devices is typically between 1and 2 times the average diameter of the smallest cross-sectional area ofthe nozzle. We have found that this droplet diameter depends also on thedegree of surface roughness in or near to the nozzle at its intersectionwith the air-side of the membrane. Such roughness may be in the form ofmicro-capillary structures on the surface, which easily wet with liquid,causing some liquid volume to reside on the air-side of the membranethroughout the hydraulic pressure cycle. In this case, the liquidmeniscus of the ejecting liquid in the positive segment of the pressurecycle is relatively poorly pinned to the circumference of the nozzle,and connects with the meniscus of the volume of liquid external to thenozzle. We have found that such a loss of control over the geometry ofthe liquid meniscus during the droplet creation process can lead to anenlargement of the droplets ejected from such nozzles and to poorcontrol over ejection direction, due to collection of additional liquidvolume from the air-side surface of the membrane. U.S. Pat. No.5,152,456 further comprise nozzle geometries without a marked and suddenchange in both cross-sectional area or in the rate of change ofcross-sectional area, as a function of the distance through thethickness of the membrane between one surface and the other. Examples ofsuch a geometry include a trumpet taper (for example formed inelectro-formed nickel—“Veconic”, from Stork Veco BV, of Eerbeck, TheNetherlands), and a conical taper (for example formed by laser drillingor etch process—“Vecoplus”, also from Stork Veco BV). Within suchnozzles, surface tension, viscous drag and hydraulic pressure forceswill dominate the liquid flow through such nozzles.

When the hydraulic pressure in the liquid is less than the ambientpressure, such nozzle geometries have the disadvantage that at the pointduring the vibration cycle and when the liquid associated with thenozzle is accelerated towards the bulk liquid, the fluid meniscus withinthe nozzle may relatively easily travel through the length of the nozzleand towards the bulk liquid such that the nozzle becomes partially orfully air-filled. Therefore, such geometries use additional hydraulicenergy in both the negative and the positive pressure segments of thevibration cycle to overcome the viscous drag associated with refillingthat nozzle from the bulk liquid in each cycle before it can generate adroplet from the liquid meniscus at the air-side. To prevent excessivemeniscus travel within the nozzle during the periodic pressure cycle, amarked and sudden increase in the rate of growth of the cross-sectionalarea of the nozzle is advantageous, by providing a potential energybarrier to the liquid meniscus as it moves along the length of thenozzle (that is; the surface area of the liquid meniscus must increasemore rapidly to overcome the discontinuity in cross-sectional areawithin the nozzle). In the same way and on the positive segment of thehydraulic pressure cycle, liquid contained within the nozzle and behindthe pinned meniscus will quickly refill the small air-filled portion ofthe nozzle to generate a droplet from the air-side, without losing muchhydraulic energy through viscous drag as it does so.

In our WO-A-95/15822 we disclose an alternative method for generatingdroplets that is based on capillary-waves. In this method, theorientation of the nozzle geometry is reversed relative to the apparatusof U.S. Pat. No. 5,152,456. Instead, the smallest cross-sectional areaof the nozzle is located at the surface of the membrane to which thebulk liquid is introduced (liquid-side). The cross-sectional area of thenozzle increases through the thickness of the membrane away from thissurface and towards the opposite surface (air-side). As before, atambient hydrostatic pressure, the liquid meniscus will move to aposition of minimum energy where the cross-sectional area of the nozzleis smallest. In this device, the nozzle is not liquid filled, rather thenozzle is substantially air-filled.

As before, during operation a periodic bending-mode vibration isgenerated in the perforate membrane, harmonically displacing themembrane towards and away from the bulk liquid, resulting in aharmonically varying hydraulic pressure in the liquid near to theliquid-side of the membrane. In addition to this harmonic pressure, theambient hydrostatic pressure in the bulk liquid (the pressure thatexists in the absence of the vibrationally induced harmonic pressure) isreduced relative to the air pressure at the opposite surface of themembrane. In this way the fluid meniscus is prevented from migratingalong the nozzle under the influence of the varying hydraulic pressure,and is usually thereby maintained at the smallest cross-sectional areaposition. The harmonic hydraulic pressure is believed now to actdirectly on the fluid meniscus to generate a surface capillary wavewithin each meniscus. This capillary wave is believed to be centredwithin the circumference of the meniscus and to oscillate in thedirection normal to the meniscus to create a capillary wave crest (cusp)at the centre of the nozzle. When liquid near to the cusp of thecapillary wave has sufficient vibrational momentum, it a droplet isgenerated with a characteristic diameter of approximately λ_(c)/3 whereλ_(c) is the capillary wavelength defined in the following expressions:ρω_(c) ²=σk³ $k = \frac{2\pi}{\lambda_{c}}$where ρ is the liquid density, ω_(c) is the angular frequency of thecapillary wave, σ is the surface tension of the liquid meniscus, and kis the wave number.

In WO-A-95/15822, the preferred condition is that the frequency of thecapillary wave is selected from the equations above, such that thediameter of the liquid meniscus within each nozzle is the same as thecapillary wavelength, and thereby the droplet ejected from the cusp ofeach capillary wave has a diameter which is smaller than the diameter ofthe smallest cross-sectional area of the corresponding nozzle.

Other operating conditions are also found to be satisfactory, includingthose in which the oscillating frequency of the perforate membrane islower than that required to produce capillary waves of angular frequencyω_(c) (see for example our WO-A-00/47334).

In such devices, the droplet must transfer through the nozzle from theliquid-side to the air side in order to create an aerosol droplet.Successful transfer of this liquid droplet requires that the capillarywave ejects the droplet along the long-axis of the nozzle to minimisethe risk of this droplet impacting on the inner surfaces of the nozzle.

These devices have the disadvantage that if such impact-occurs withinthe nozzle, then at least part of the liquid contained within theimpacting droplet deposits upon part of the inner surface of the nozzle.Then a catastrophic failure of droplet ejection can occur. In order toreduce the risk of droplets impacting within the nozzles of suchdevices, it is advantageous to provide nozzles with large rates ofincrease of cross-sectional area between the liquid meniscus and the airside of the membrane.

Nozzles with large rates of increase of cross-sectional area between oneside of the membrane and the other are difficult to fabricate sincerelatively small errors in the depth of any forming process from nozzleto nozzle will result in large errors in the cross-sectional area at theintersection between the nozzle and either surface of the membrane (thatis, the nozzle aperture). For example, such errors will causesignificant variations in the shape and size of the liquid meniscuswithin the nozzle, arising from the diameter of the meniscus. This willresult in a dispersion of capillary wave frequencies across the singleperforate membrane, thereby reducing the number of nozzles supportingcapillary waves whose optimum excitation frequency is well matched tothe vibration frequency of the perforate membrane. Therefore, to enablea nozzle fabrication process to create a nozzle with a large rate ofincrease of cross-sectional area between the liquid side of the membraneand the air side of the membrane for such devices, it is greatlypreferred to have a portion of the nozzle having a slow rate of changeof cross-sectional area in the region of the intersection between thenozzle and the liquid-side of the membrane.

Therefore, for both the devices of the types described in U.S. Pat. No.5,152,456 and those described in WO-A-95/15822 and WO-A-00/47334, it isdesirable to provide nozzles having a portion with a slow rate of changeof cross-sectional area and another portion having a rapid rate ofchange of cross-sectional area, and preferable for the transitionbetween those two portions to be marked and sudden. Perforate membranesare also used to create fluid (meaning liquid or gas) pumping devices.In the general case fluid is transferred through the nozzles in bothdirections, but the nozzles have a resistance to flow which is not equalin both directions, resulting in a net fluid flow in one direction afterone or more complete cycles. Devices of this type are disclosed inCH-A-280 618, and WO-94/19609, wherein tapered nozzles in a chamberwall, plate, or membrane are subjected to cyclic fluid flow in bothdirections through these tapered nozzles, but resulting in a net forwardpumping effect.

In CH-A-280 618, it is disclosed that a perforate plate containingtapered nozzles is displaced forwards and backwards in a fluid-filledchamber, and alternatively that a perforate plate is fixed and aseparate diaphragm positioned within the walls of the fluid chamberdrives the fluid forwards and backwards through the nozzles in theplate. The flow of fluid through the nozzles in this reciprocating plateis restricted by unequal turbulence effects known elsewhere(WO-94/19609) as ‘nozzle’ and ‘diffuser’ flow. Such unequal turbulenceeffects result in a net transfer of fluid in the diffuser direction offluid flow, wherein the cross-sectional area of the nozzle isincreasing.

In general, such devices transfer a large total volume of fluidbackwards and forwards through each nozzle, that is significantlygreater than the net pumped volume of fluid. Such devices are thereforesubject to more viscous fluid drag than unidirectional pumps (such aspositive displacement pumps) providing an equivalent net flow. Thisrequires excess driving power to overcome these viscous losses. Viscousdrag in a laminar fluid flow within a narrow channel is characterised byPoiseuille's equation for capillary flow (Gases, Liquids and Solids, D.Tabor, 2^(nd) Edition, Cambridge University Press (1979):$Q = {\frac{\pi\quad r^{4}}{8\eta}\left( \frac{p_{2} - p_{1}}{l} \right)}$where Q is the fluid flow rate, r is the radius of the capillary (ornozzle), η is the viscosity of the fluid, and$\left( \frac{p_{2} - p_{1}}{l} \right)$is the average pressure gradient along the capillary channel. Therefore,the flow rate is strongly limited by the radius of the nozzle, andespecially by the smallest radius of the nozzle. In these nozzles thisviscosity limitation dictates that a much higher-pressure gradient isrequired to generate equivalent flow rates to those observed throughnozzles with only a slightly larger radius.

Therefore it is desirable to control precisely the radius of thenarrowest section of the nozzle geometry while also minimising thelength of the same section, in order to maximise the pressure gradient.In this way the viscous drag of the fluid flow within each nozzle willbe optimised to provide the same flow performance both from each nozzlein the membrane and from each pumping device.

Perforate membranes are also used in commonly availablethickness-absorption and surface-rejecting fluid (meaning liquid or gas)filters, where fluid is transferred through the nozzles in one directiononly. Solid particles suspended within such fluid and with all lineardimensions greater than or equal to the smallest diameter of the nozzlescontained in the membrane do not pass through that nozzle. Thus a filtermembrane of this type will remove those particles, which are larger thanor equal to the minimum diameter of the nozzle, from the fluidtransferred through the nozzles.

These devices are susceptible to two limitations, which govern theefficiency of such a filter membrane. In the first limitation, thespread in the diameter of the smallest cross-section of each nozzleshould be as narrow as possible, in order that the fluid flow througheach nozzle is subjected to the same degree of filtration. The spread innozzle diameter therefore provides a direct indication of the sharpnessof the cut-off in the diameter of particulates, which are allowed topass through such nozzles. In the second limitation, the flow rate offluid through such filter membranes is determined by the same viscousdrag as described above in relation to the fluid pump.

Therefore, it is desirable to maintain a constant radius of the smallestcross-section of each nozzle and furthermore it is also important for agiven pressure differential across the faces of the membrane, tomaximise the pressure gradient within such nozzles by minimising thelength of the smallest cross-sectional portion of the nozzle.

Various laser drilling processes are described in WO-A-99/01317,FR-A-2112586, WO-A-90/08619, U.S. Pat. No. 5,063,280, DE-A-19636429 andEP-A-0729827 in relation to the penetration of materials in a controlledmanner using laser radiation. It is also known that the geometry of suchpenetration is difficult to control accurately below 10 μm diameter. Inaerosol devices, nozzle pumps and filter membranes, small nozzlediameters are desirable to respectively achieve inhalable droplets formedical drug delivery to the nose and/or lungs, high pressure fluid pumpdelivery, and fine high quality particle filters. One reason for thisdifficulty Is the limited control of excess heating and ablation ofmaterial from the nozzle. This is commonly addressed through the use ofa photo-detector positioned either above or below the material to detectthe moment at which sufficient material has been removed from thenozzle, and controlling the laser energy in response to this detectionand thereby prevent further unnecessary material heating and ablation.In this way some control may be provided over the depth of the lasermachined feature, however this remains insufficient to also accuratelycontrol the diameter of the nozzle below 10 μm diameter at theintersection with the opposite surface of the membrane.

Another reason for this difficulty is that the thickness of the materialrequired to be penetrated by the laser beam is substantial; typically ofthe order of 25 μm to 200 μm for the aerosol generating devices,nozzle-plate pumps and filters described above. However, many of theapplications of these devices also require 10 μm diameter or smallernozzles, which would result in aspect ratios (of minimum nozzle diameterto membrane thickness) of between 2.5 and 20. Known thermal laserdrilling techniques, especially those used with metallic membranes, maybe controlled to produce only limited aspect ratio features (usually<3). For example, aerosol devices, nozzle pumps, and filter membranes;formed with in such aspect ratio limitations will suffer from lowmembrane stiffness, and cannot generate (or withstand) the amplitude ofthe operating pressures described above that are desirable for theireffective operation. In order to make a droplet from an aerosol device,whose diameter is such that the droplet is respirable, then the nozzlediameter must be less than ø10 μm. By known thermal laser drillingtechniques, this membrane must be less than 30 μm thick, and membranesof such thickness are found not to be robust in use.

However, forming nozzles in the manner suggested in the prior art usinglaser drilling techniques leaves the nozzles with relatively coursesurface finishes and therefore it is desirable to polish or smooth thesurfaces. A typical way of doing this would be to electro-polish them,but, with tapered nozzles this would involve removing material from theinternal surface of the nozzle with the result that the diameter of thesmaller aperture of the nozzle is increased beyond the desired value, asthe electro-polishing process removes material generally normal to thesurface within the nozzle at any point, making it extremely difficult tocontrol the diameter unless the shape is known very precisely and thecontrol is achieved also very precisely.

There is a need therefore for a process which can overcome thisdifficulty.

According to the present invention there is provided a method of formingperforate membrane for use in a liquid transport device, by applyinglaser energy selectively to a first surface of the membrane in the formof a pulsed, focused beam to form a plurality of nozzles each having athroat portion opening at one end through the opposite surface of theperforate membrane and a smoothly curved outwardly diverging portionextending from the other end of the throat portion to the first surfaceof the perforate membrane, characterised by

-   -   thereafter electro-polishing the first surface of the membrane        and the surface of the diverging portion of the nozzles to        remove surface imperfections, and controlling the        electro-polishing so as to remove material from the surface of        the diverging portion of the nozzles to a depth less than the        length of the throat portion.

Because of the presence of the throat portion, essentially narrower thanthe diverging portion, removing material from the diverging portion bythe electro-polishing process substantially affects only the length ofthe throat portion, so that the diameter remains substantiallyunaffected.

The invention also includes a perforate membrane manufactured by such aprocess and a fluid transport device including such a membrane.

The laser energy may be applied in two steps to form the nozzles,between which steps the distance between the laser focus and the firstsurface of the membrane and/or the pulse energy of the laser beam isadjusted.

The nozzle described in the present invention contains such a throatportion with a relatively constant cross-sectional area, extendingbetween the opposite surface of the membrane and a diverging portion ofthe nozzle that intersects with the first surface of the membrane. Thisprovides a reliable and repeatable throat diameter, and also provides arelatively short throat portion compared to the thickness of themembrane, thereby increasing the pressure gradient along the throatportion of the nozzle and in so doing this will limit the effects ofviscous drag. The throat portion dominates the viscous flow even throughrelatively thick membranes, since the diverging portion passes fluidrelatively freely to the throat portion because it has a cross-sectionalarea which is always greater than the throat portion. Therefore, thisnozzle provides a flow channel, which provides a method for optimisingthe viscous drag associated with such fluid pumping devices as disclosedin CH-A-280 618 and WO-94/19609.

We have developed a percussion laser drilling process that addresseslimitations of other laser processes, to create a nozzle of controlleddiameter and taper. This process may be employed on a wide range ofmembrane materials, including metals, ceramics, glass and polymers. Inaddition this new process enables high-speed automatic focus controlnecessary for manufacturing these perforate membranes in high volume.

In summary, this method operates as follows:

-   -   a. Provide a focused laser spot, such that the distance between        this laser spot and the first surface of the membrane can be        altered. The optical axis contains the positions of maximum        laser energy density at all points along the focused portion of        the laser beam. The optical axis is arranged such that it is        incident to the membrane at the desired angle of the nozzle to        the membrane surface, which is usually (but not necessarily) 90°        for planar membranes. This angle may be different from 90°        particularly where the membrane is non-planar.    -   b. Position the membrane material such that the laser focus will        fall either above or below the thickness of the membrane. By        changing this distance along the optical axis between the first        surface of the membrane and the laser focus position, we can        control the area of the illuminated surface of membrane.    -   c. Illuminate the first surface of the membrane with pulsed and        focused laser radiation with a fluence in excess of the material        ablation threshold over a controlled surface area. Material is        thereby removed from the thickness of the material through and        below the illuminated surface. After a predetermined number of        pulses, known by prior experiment or otherwise to be        insufficient for the laser beam to penetrate through the        thickness of the material, the laser radiation is switched off.        This step forms the diverging portion of the required nozzle.    -   d. Reduce the distance between the laser focus position and the        first surface of the membrane by a predetermined amount along        the laser axis such as to reduce the area of the surface        illuminated at a given illumination intensity in accordance with        the cross-sectional area of the required nozzle at the interface        between the diverging portion and the throat portion of the        nozzle. Also adjust the pre-set laser pulse energy such that the        laser fluence over this smaller surface within the nozzle will        be approximately equivalent to that used to create the diverging        portion.    -   e. Illuminate the membrane for a predetermined number of pulses        to remove the remaining material thickness within the throat of        the required nozzle. The pulse at which laser light first        penetrates through the whole thickness of the material is        detected by a photo-detector positioned on the opposite surface        of the membrane.    -   f. At the same or similar settings, further laser pulses are        applied to the nozzle after the first penetration pulse to clear        unwanted debris from the nozzle intersection with the opposite        surface, to create a substantially round and smooth        cross-sectional area of the nozzle.    -   g. The number of laser pulses required to first penetrate the        through the throat of the nozzle N_(t) is compared to the number        of pulses predetermined for the creation of such portion N_(set)        in the following way:        N _(t) −N _(set) =N _(error)    -   h. If N_(error) is greater than 0, then there was an        insufficient membrane material removal rate during the formation        of this nozzle. In the absence of other uncontrolled effects,        this is usually due to small variation in the set distance        between the initial laser focus position and the first surface        of the membrane, relative to that optimised for the        predetermined laser settings.

To correct for this variation, the distance between the initial laserfocus position and the membrane surface is reduced by a small andpredetermined amount. Similarly, if N_(error) is less than 0 then thedistance between the initial laser focus position and the membranesurface is increased by a small and predetermined amount.

This method results in the formation of the nozzle geometry describedabove, in which the diverging portion of the nozzle is formed firstthrough a predetermined portion of the thickness of the membrane. Thethroat portion is then formed to connect the diverging portion of thenozzle to the opposite surface of the membrane through the remainingthickness of the material.

The divergence of the diverging portion of the nozzle arises from thenature of the laser ablation process. This process (which is a complexprocess and imperfectly understood) is now described.

The laser radiation transfers energy into the surface of the material.This energy causes highly localised heating of the membrane material onand under and around the illuminated surface. If the laser power densityis sufficiently high, then direct ablation of the material occurs at themembrane surface. At a lower power threshold, the laser energy transfersinto thermal energy in both the molecular structure and electrondistribution (especially in the case of metals) within the material,resulting in the formation of a localised molten pool of material(thermal melt). Below this lower power threshold the laser energy willcause mechanical and structural damage to the material as the locallaser heating anneals and deforms the material, but will not remove it.

During the ablation process, material is removed more quickly from areasof the illuminated surface where the laser power density (fluence) ishighest. At the initial pulse, this will match the profile of theincident laser power density over the surface of the membrane. In afocused laser beam, this power distribution may be approximated to aGaussian profile, thereby concentrating the ablation towards the centreof the illuminated surface. As the ablation process proceeds through thethickness of the material, a curved surface begins to form whereby thereis a smooth gradient between the deeper centre and the perimeter of theilluminated surface. As the ablation process moves deeper into thethickness of the membrane, and after a predetermined number of incidentlaser pulses, the diameter of the illuminated membrane surface and theincident laser power density is changed by relative adjustment of thelaser focus position and the laser pulse energy. This results in eithera reduction, or an increase in the ablation rate as a function of theablation depth, depending on whether the ablating surface is moving awayfrom or towards the laser focus position, respectively. For betterprocess stability, the first surface of the membrane is preferablypositioned beyond the laser focus position in order that the ablationrate naturally reduces as a function of ablation depth to form thediverging portion. In this way it is found that better control may beexercised over the depth of the diverging portion, and thereby over theposition of the interface between the diverging and the throat portionsof the nozzle within the thickness of the membrane material.

In the case of metallic membranes, the wall of this curved surfacebecomes lined with thermal melt, which solidifies to form a relativelysmooth re-cast melt layer. Furthermore, the smooth walls now begin toreflect at least some of the incident laser radiation towards the centreof the curved surface. This further increases the material ablation rateat the centre of the illuminated surface. Above the surface, the highenergy of the ablation process forms a plasma. This plasma has theeffects of scattering and absorbing some of the incident laserradiation, thereby distribution thermal energy over the membranesurface. The pressure within the plasma also drives some of the lasermelt away from the inside of the curved surface at the ablation site,from where it flows into a radially expanding distribution of thermalmelt which re-casts at and beyond the intersection of the divergingportion with the first surface of the membrane.

These processes described above result in a curved and diverging cavityin the first portion of the membrane thickness wherein thecross-sectional area of the cavity reduces between the first surface andthe base of the cavity within the thickness of the membrane. This cavitywill approximate to a part-spherical profile in general to reflect theincident laser beam profile, and the distribution of re-cast melt aroundits circumference.

The bottom of this diverging portion is substantially flat with atangential plane being parallel to the plane containing either surfaceof the membrane. This ensures that there is a relatively well definedsurface through which the throat portion of the nozzle may be formedthrough the remaining thickness of the membrane. In the method describedabove, the throat portion is formed using the portion of the laser beamnear to the laser focus position. The laser beam profile near to thelaser focus position is limited by diffraction to form a curved waistbetween the converging and diverging portions of the beam, rather thanthe sharp point implied by a simple linear ray diagram. This waistfeature provides a relatively slowly changing beam profile at variouspositions orthogonal to the beam axis and near to the laser focusposition. Therefore, as the laser beam is used to form the throatportion through the remaining thickness of the material, the laser pulseenergy remains substantially un-changed. Additional laser pulses areapplied after the first pulse to penetrate the throat portion isdetected by a photodetector positioned on the opposite side of themembrane to the incident laser beam. This process results in arelatively smooth-walled and slowly varying cross-sectional area alongthe length of the throat portion.

The number of pulses, as detected by the photodetector, required topenetrate the throat portion to form the preceding nozzle is used tocontrol the distance between the initial laser focus and the firstsurface of the membrane for the subsequent nozzle to be formed at anadjacent position. This method is practised by the inventors forcontrolling the initial focus position of the laser beam at a constantdistance from the first surface of the membrane, thereby correcting fornormal variations in the flatness of the membrane. This focuscontrolling process is integrated within the laser drilling process andthereby presents no additional step or time penalty to execute suchprocess. In this way, the laser focus may be accurately controlledrelative to the surface of the membrane in a high speed manner (which isdesirable when forming many nozzles in each membrane, for example toenable high volume manufacturing of such perforate membranes).

Especially in the case of metal membranes, and resulting from thethermal nature of the laser process used to form each nozzle, re-castmelt is deposited on the inner surface of the nozzle and on bothsurfaces of the membrane surrounding each nozzle. This re-cast melt hasa capillary-like structure which causes unwanted fluid flow over thesurface of the membrane, resulting especially in uncontrolled dropletcreation from aerosol devices. Also, the brittle nature of the re-castmelt presents a significant risk of fracture of some of this materialduring device operation. This can result in potentially dangerousparticulate contamination in the fluid delivered by the fluid transportdevice.

Particularly in the case of metallic membranes, in order to remove theunwanted re-cast melt from the surface of the membrane and from withinthe nozzles, it is desirable to use a cleaning process prior to thecompletion of the membrane manufacture. We have found thatelectropolishing provides such a process.

The electropolishing process is well known in the metal finishingindustry, and can be referenced as a standard process, see for exampleL. J. Durney, Electroplating Engineering Handbook, Part 1, Ch.3-D,Fourth Edition, Chapman & Hall, NY, 1996, and J. Brown, AdvancedMachining Technology Handbook, Part VII, ISBN 007008243X, 1998.Electropolishing provides a high quality surface finish to a range ofmetal surfaces including stainless steel, titanium, nickel, gold,Hastalloy, copper, bronze, brass, beryllium-copper alloys and aluminium.The electric field is applied between an anode to which the metalcomponent (membrane) is attached and a cathode, immersed together in aliquid electrolyte solution. At the surface of this component, theelectric field lines impinge at right angles to the conducting surface.At regions of the surface where the radius of curvature is small, theelectric field gradient becomes very steep, attracting a higher rate ofdielectrophoretic migration of the electrolyte towards these regions. Asa result of this, these regions are etched most strongly by theelectrolyte, to remove metal ions from the sharp surface features on themembrane surface. This preferential material removal acts to increasethe radius of curvature of the metal surface, and thereby smooth thesurface by removing sharp features. Advantageously, we have defined anew method of electropolishing (see description related to the FIG. 5)in which the electropolishing may be applied selectively to each side ofthe membrane, thereby preventing over-etching of the finer details ofthe nozzle geometry, in particular the diameter of the throat portion.

The inventors have found that conventional electropolishing isinadequate to remove undesirable re-cast melt formed by the above laserdrilling process; but have also found a new method of electropolishingrelated to the specific geometry of the nozzles claimed herein issuccessful at removing that undesirable re-cast melt whilst preservingthe desirable features of the laser drilled nozzle geometry describedabove.

Thus, the nozzle geometry contains at least two portions distributedthrough the thickness of the membrane. The both portions may besubstantially circular in cross-section, characterized by varyingcross-sectional area.

The first portion is preferably a substantially cylindrical geometryintersecting the opposite surface of the membrane at one end. This firstportion is known as the throat of the nozzle. At the opposite surface,the intersection with the throat portion provides a well defined andsubstantially circular opening which usually contains the narrowestcross-sectional area of the whole nozzle. In some cases the throatportion will also contain a small increase in cross-sectional areathrough the membrane thickness in the direction away from the oppositesurface.

The second portion is characterised by a diverging cross-sectional areathrough the thickness of the membrane, connecting to the other end ofthe throat portion and diverging in the direction between the other endof the throat portion and the first surface of the membrane. The throatportion and the diverging portion of the nozzle are substantiallycoaxial.

At the intersection between the throat portion and the divergingportion, the cross-sectional areas are continuous, however the rate ofchange of the cross-sectional area as a function of distance through themembrane thickness shows a sudden and marked change at the intersection.Therefore at this intersection, a step exists between the two portionsand within the thickness of the membrane.

More than two portions may also be present in such nozzles. In that casea number of diverging portions are distributed between the other end ofthe throat portion and the first surface. These diverging portions aredistributed in order of increasing cross-sectional area between theother end of the throat portion, and the first surface of the membrane.The cross-sectional areas at the intersection of all portions within thethickness of the membrane are continuous. In the same way as in thetwo-portion nozzle, the rate of change of cross-sectional area, as afunction of distance through the membrane thickness, shows a sudden andmarked change at the intersection between the throat portion and thefirst diverging portion connected thereto.

One example of a membrane and its method of manufacture according to thepresent invention will now be described with reference to theaccompanying drawings, in which:—

FIG. 1(a) to 1(g) are drawings of variations in the geometry of nozzlesin a membrane;

FIG. 2 is a schematic of a laser apparatus for creating nozzle aperturesin a membrane;

FIG. 3 is a partial cross-sectional view of the membrane illustratingsteps in the process of manufacture;

FIG. 4 illustrates the membrane cross-section before and afterelectropolishing.

FIG. 5 is a cross-sectional view through an electropolishing system usedin the process of the invention;

FIG. 6 is a chart illustrating the spread of pulses required to form thethroat portion of nozzles in a single membrane, when the target numberof pulses is pre-set to 11;

FIG. 7 is a chart illustrating the spread of pulses required to form thethroat portion of nozzles in a single membrane, when the target numberof pulses is pre-set to 15;

FIG. 1(a) shows, in plan view, a portion of the central perforated areaof membrane 1, which is a flat disk typically made of AISI 316 or AISI302 stainless steel and is of thickness 50 μm and of overall diameter 12mm (not shown). This central perforated area may be 8 mm in diameter(not shown) within which nozzles are evenly distributed in a triangularpattern. The nozzles 10 are separated by a distance 4, which iscontrolled to achieve the required net fluid flow rate through theperforate membrane. For example, this distance may be between 40 μm and5001 μm separation and more usually is set to 100 μm, which translatesto more than 5,800 nozzles within each membrane.

FIG. 1(b) shows the geometry of a nozzle 10 contained in a perforatemembrane 1 according to the present invention contain two primaryportions, a throat portion 12 and a diverging portion 11. Note thatthese two portions are concentric.

FIGS. 1(c), 1(d), and 1(e) illustrate cross-sectional views throughthree examples of variations of the profile of the diverging portion 11along the chord 13 of FIG. 1(b). In FIGS. 1(c) and 1(d), the length,diameter and taper of the throat portion 12 remain substantiallyunchanged. However in FIG. 1(e) the throat portion 12 has been shortenedwithin the fixed thickness of the membrane to accommodate a largerdiverging portion 11. Such changes may be controlled by suitableadjustment of the pre-set laser drilling parameters, as discussed belowin relation to FIGS. 6 and 7.

FIG. 1(c) shows the diverging portion 11 with a frusto-conical profileresulting from a constant increase in cross-sectional area. FIG. 1(d)shows the diverging portion 11 with a smoothly curved, outwardlydiverging portion resulting from a maximum rate of change ofcross-sectional area at the intersection between the throat portion 12and the diverging portion 11, where this rate of change reduces towardszero at the intersection between the diverging portion 11 and the firstsurface. FIG. 1(e) shows a diverging portion 11 that penetrates deeperwithin the membrane thickness, thereby shortening the throat portion 12.Such variations are important for controlling the viscous drag ofdifferent fluids within such a nozzle, for example contained within anaerosol device.

FIGS. 1(f) and 1(g) illustrate variations on the nozzle geometryachieved through multiple laser drilling steps to form the divergingportion 14 and 15 respectively. We have found that the diverging portionmay be altered to change the distribution of rates of change ofcross-sectional area. For example, FIG. 1(f) shows a diverging portion14 where the rate of change of cross-sectional area is greatest towardsthe intersection between the diverging portion and the first surface, toform a trumpet-shaped taper. Such a diverging portion may be formedusing the laser drilling process to create 4 steps of the laser powerdensity and/or of the illuminated surface area. Note that the throatportion 12 maintains the discontinuity of the rate of change ofcross-sectional area at the intersection with the diverging portion 14.

Similarly FIG. 1(g) shows how multiple steps used in the formation ofthe diverging portion 15 may be used to form customised variations ofthe rate of change of cross-sectional area to suit particular deviceapplications. Note that in this case also, there is a discontinuity inthe rate of change of cross-sectional area at the intersection betweenthe throat portion 12 and the diverging portion 15.

The smallest diameter of the throat portion, which is usually at theintersection between the throat and the opposite surface, may becontrolled at least in the range between 1.5 μm and 30 μm, and, fordroplet generating devices, is more usually set to between 2.5 μm and 3μm. For membranes for use in such devices, the diameter of the divergingportion 11 at the intersection with the first surface may be between 101μm and 50 μm, and is more usually set to between 35 μm and 40 μm. Theratio between the length of the throat portion to the length of thediverging portion of the nozzle may be between 1 and 0.3, and moreusually is set to 0.5, in which case the length of the throat 12 isapproximately 171 μm and the length of the diverging portion is 331 μm,through a membrane whose thickness is 50 μm.

A laser drilling method is used to create these nozzles, whereby thegeometry of the diverging portion substantially conforms to a partspherical profile, as shown in FIG. 1(d). The laser drilling process iscarried out using the laser apparatus shown in FIG. 2, which includes anX30-532QA diode-pumped Nd-YAG laser head 20 driven by a T40-8THHSS40power unit (not shown) (both supplied by Spectra Physics Lasers, Inc.,1330 Terra Bella Avenue, Mountain View, Calif. 94043, USA); a Pockel'scell modulator 21 (Model: LM0202 P5W, Linos Photonics GmbH, ofGoettingen, Germany); a computer controller 25; an x-y plane translationstage 29, and a z-plane translation stage 28 (‘Physik InstrumenteM-125-11’ from Lambda Photometric Ltd. of Harpenden, UK); suitable beamsteering optics 22 which are all standard equipment (supplied by ElliotScientific Ltd. of St. Albans, UK); an objective lens 23 which is a 14mm (NA=0.17) microscope objective; a photodiode 26 located below thesample stage 24 as shown and which comprises a BPX65 fast responsephotodiode (from RadioSpares); and a suction tube 27 provided to removeablated material.

The controller 25 controls the laser head 20, the Pockel's cell 21, andthe x-y-z translation stages 29, 28, and receives signals from thephotodiode 26. The process of drilling the nozzles within the membraneis discussed more fully in relation to FIG. 3, below.

FIGS. 3(a) to (f) illustrate the laser drilling process used to createthe nozzles 25-10 in the membrane 1. FIG. 3(a) shows a schematic of thefocus of the Gaussian laser beam, as generated by the objective lens 23.At the focus, the distribution of photon energy becomes diffractionlimited, giving rise to a curved beam waist 31 rather than a singularfocal point. The following relation gives the width of the beam waist:$\alpha_{0} = \frac{2\lambda}{NA}$where NA is the utilised numerical aperture of the lens 23, and λ is thewavelength of the incident laser light. In this case, NA=0.17 and λ is532 nm, therefore α₀ is approximately 6.3 μm.

Notice that the smallest cross-sectional area at the laser focus 31 isat the focal length 32 of the objective lens 23. Notice also that thedistribution of photon energy is symmetrical both above and below thelaser focus.

To form nozzles, the controller cycles through the settings illustratedin FIGS. 3(b), (c), (d), (e) and (f). In the step shown in FIG. 3(b),the controller positions the z-stage 28 such that the laser radiationwill illuminate (at a given intensity) a certain area of the membrane'ssurface. In this case, the membrane 1 is positioned with its uppersurface 2 a certain distance 33 below the objective lens, such that thelength 33 is greater than the focal length 32. Once the z-stage 28 is atthe correct position 33, the Pockel's cell 21 is triggered to allow anumber of laser pulses through the steering optics 22 to illuminate thesurface 2, as illustrated in FIG. 3(c). Each pulse is 10 ns in duration,and contains 532 nm wavelength laser radiation, and with a peak energyof 10 mJ per pulse. After a pre-set number of pulses have passed throughthe Pockel's cell 21 (usually 20 pulses), as counted by the controller25 in response to the pulse triggered signal from the laser 20, thePockel's cell 21 is used to extinguish the transmitted laser radiationto the membrane 1. This first step results in the formation of thediverging portion 11 of the nozzle 10 by the process of laser ablation,to form a part-spherical geometry through approximately {fraction (2/3)}of the membrane thickness.

Following this first laser drilling step, the controller 25 moves thez-stage 28 to a new distance 35 between the objective lens 23 and thesurface of the membrane 1, as shown in FIG. 3(d). Alternatively or inaddition, the controller 25 sets a lower peak laser pulse power throughthe Pockel's cell 21. When all these conditions are set, the controller25 then triggers the Pockel's cell to deliver a series of pulses throughthe steering optics and onto the surface of the membrane 1, to commenceablation of the throat portion 12, as illustrated in FIG. 3(e). Again,these pulses are 10 ns in duration, at 532 nm wavelength, however nowwith a reduced peak energy of 15 μJ per pulse.

During this series of pulses, the controller 25 counts the number ofpulses delivered to the membrane surface 2, and at the same timemonitors the output from the photodiode 26 (not shown). The pulse countat which the photodiode 26 first detects a pre-set increase in opticalpower is used by the controller to determine the number of pulses usedto penetrate the throat portion 12 of that nozzle 10. This number isthen compared to a pre-set target number (usually 11 pulses in thisexample) to determine the error associated with the drilling processused to create that nozzle. Following the first pulse detected by thephotodiode 26, the controller 25 delivers a further pre-set number ofpulses through the Pockel's cell 21 to the membrane 1. This is done inorder to fully form the throat portion 12 (usually 10 pulses in thisexample) and to create a substantially circular cross-section within thethroat portion 12 at the intersection between it and the surface 3 ofthe membrane 1. Following this series of pulses, the throat portion 12of the nozzle 10 is fully formed through the remaining ⅓ of thethickness of the membrane, as illustrated in FIG. 3(f). The stepillustrated in FIG. 3(e) is controlled to obtain the desired minimumcross-sectional area of the nozzle 10.

The error associated with the drilling process is used to determinesmall variations in the rate of ablation of the membrane material. Thisvariation is usually due to variations in the laser fluence incident onthe surface of the membrane. When the delivered laser power isstabilised, changes in laser fluence only occur due to changes in theilluminated surface area of the membrane 1. In a focused laser system,such changes are due to variations in the distance between the laserfocus position and the membrane surface 2, due to errors in membraneflatness. Variations in the membrane height over the nozzle drillingarea are corrected by adjusting the distance between the membranesurface and the laser focus position, by adjusting the z-stage 28. Thisadjustment is made in response to the error associated with the drillingprocess in the following way:N_(t) −N _(set) =N _(error)N_(error)=0; δz=0 μm,N_(error)>0; δz=+7 μm,N_(error)<0; δz=−7 μm.where δz is measured in the same direction as the z-stage 28 motion inthe step between the ablation of the diverging and the throat portionsof the nozzle.

In this case, the variation in the gap is controlled using a simplealgorithm, with a linear step of 7 μm in the z-axis. It is recognisedthat this algorithm may be developed further, for example to incorporateproportionality between the magnitude of N_(error) and the correctionδz. However, we have found that this simple algorithm provides adequatecontrol over the pulse count, and thereby control of the nozzlegeometry. For example, this algorithm, in combination with the methoddescribed above, may be used to create a membrane containing 5800nozzles, each to within ±0.2 μm error of a target diameter of 2.8 μm forthe throat portion diameter. It is noted also that this process providesa suitable method for controlling the gap between the laser focusposition and the membrane in order to enable high-speedmembrane-manufacturing rates that are within reasonable commerciallimitations. For example, each membrane component containing 5800nozzles may be drilled in less than 100 seconds.

This apparatus has been described with reference to drilling holes instainless steel, and may be applied in a similar manner to a wide rangeof metals including aluminium, brass, copper, Constantan, Hastalloy,nickel, niobium, titanium, tungsten, tantalum, Waspalloy, zirconia. Itis clear that a similar predetermined increase in laser transmissionthrough the throat 12 of the nozzle 10 is detected by the photodiode 26and may be used to control the drilling process with less opaquematerials such as plastics, glass and silicon. In such materials, it isalso clear that different laser sources (i.e. at a different wavelengthof light) may be used, for example an Excimer laser at 192 nm to 351 nmwavelength may be used to ablate plastics, glass, silicon.

FIGS. 4(a) & 4(b) illustrate the effect of electro-polishing to removethe re-cast thermal melt and other debris from within and around thelaser drilled nozzle. The recast melt 40 is shown in FIG. 4(a) where itis substantially localised into two annular features near to the upperrims of the diverging portion 11 and the throat portion 12, of thenozzle 10, known as crowns. Over the remaining surfaces within thenozzle 10 a thin layer of recast melt 41 is distributed into a rippledstructure to reflect the pulsed nature of the ablation process used tocreate such nozzle. On the upper (first) surface 2 of the membrane thereis an expansive and radial distribution of recast melt 42 which isloosely connected to the crown on the diverging portion 11 near to thenozzle exit, and with a disconnected distribution of recast meltfragments further from the nozzle. FIG. 4(b) shows the nozzle 10 afterpost-processing by electro-polishing, in which the recast melt has beenremoved from within and around the laser drilled nozzle. Also a quantityof membrane material is removed from a thin layer at the oppositesurface 2′ of the membrane 1, and also within the nozzle 10. This layerthickness is carefully controlled to preserve the geometry of the nozzle(usually less than 1 μm thick).

As a preliminary finishing step, the membrane 1 is degreased using anon-chlorinated solvent. An acid-solution is used to clean the membrane1 to remove the carbonised outer surface of the laser drilled nozzle,for example a solution of Nitric-HF (10% HNO₃, 2% HF) at between 50° C.and 60° C. An alternative pre-cleaning process that has been used is toheat anneal the stainless steel membrane 1 to approximately 1060° C. ina vacuum, then rapid quench the membrane in air, water, or oil at roomtemperature. This process has the advantage of releasing any residuallocal stresses created in the membrane 1 through the drilling process.Additionally, and in the same way as the Nitric-HF pre-cleaning method,rapidly quenched membranes shed the brittle and carbonised outer surfaceof the nozzles due to thermal shocks and brittle fracture.

After any of the above pre-cleaning processes, the laser drilledmembrane surface regains a metallic appearance to the naked eye. Thesurface of the membrane around and within the nozzles now comprisesrecast thermal melt that is relatively firmly connected to the surfaceof the nozzle. This remaining material may now only be removed bymechanical abrasion or focused chemical etch processing such aselectro-polishing.

FIG. 5 shows a schematic layout of the electro-polishing apparatus usedto post-process the membranes 1 after laser drilling and pre-cleaning.Advantageously, and due to the asymmetry of the nozzle geometry, thetitanium anode 50 has been modified to position the membrane 1 such thatthe electro-polishing etch is directed only to one side of the membraneat any time. A plug 53, made from PEEK, seals the rear surface 2 of themembrane 1 from the electrolyte 56. The side of the anode 50 facing thetitanium cathode 51 has an aperture 54 cut in such a way to expose thefront surface 2′ of the membrane 1 to the electrolyte, and the electriccurrent generated between the anode 50 and the cathode 51. Using thismodification, the etch processes may be applied wholly to the frontsurface 2 of the membrane 1, thereby controlling the quantity ofmaterial removed from that surface in order to maintain all of the finedetails of the nozzle geometry.

In general, for aerosol droplet applications, the ‘air-side’ 2 of themembrane should be cleaned to improve droplet ejection control.Therefore, at least that side of the membrane should be exposed to theetchant electrolyte. The anode 50 and the cathode 51 are immersed in theelectrolyte solution 56, usually comprising oxalic or phosphoric acidusually stabilised at 80° C. Alternatively, a mixture of 36% sulphuricacid, 50% glycerine and 14% water may also be used. In order to maintaina homogeneous solution the electrolyte is usually circulated with amagnetic stirrer (not shown).

The electrical power source 55 provides a constant current in thecircuit between the cathode 50 and anode 51, and through the electrolytefor a pre-determined period of time as shown by the arrow 55. Thiscurrent flow removes a controlled quantity of metal ions from the anode50 and from the exposed membrane surface, while at the same time,cations are deposited on the cathode 51. The removal of ions from themetal surface 2 of the membrane 1 is greatest in the regions of thatsurface where the electric field gradient is highest. In this way it isbelieved that the ionic species responsible for the etching processwithin the electrolyte are able to migrate to these regions underdielectrophoretic migration and that these species are thereby able toovercome the build up of the charge double layer in the electrolytewithin the gap between the electrodes. These regions of high electricfield gradient near to the surface are associated with conductingsurface structures where the geometric radius of curvature is smallest.This electric field ‘focusing’ on the regions with the smallest radii ofcurvature first, eventually reducing these features to leave resultingin a flat and highly polished metal surface.

The quantity of charge removed from the surface 2 can be related to anaverage depth of material removed from the surface 2 of the membrane.For membranes described in this application, typical current values areset to between 75 mA and 240 mA for between 50 seconds and 100 seconds.When combined with the surface area of the exposed membrane, thisrelates to a total charge removal of between 0.15 C/mm² and 0.25 C/mm².If we assume that the ions removed are all charged with a valance of 2+(e.g. Fe²⁺ ions), and that the packing density of atoms within thestainless steel surface 2 is approximately 60%, then this removalequates to an average thickness of 0.1 μm is removed from the exposedfront surface 2 of the membrane. However, the focusing effect of theelectric field gradient will result in a distribution of etch ratesacross the surface 2 of the membrane 1. For nozzles created in AISI 316stainless steel membranes, where a single-sided electro-polish isapplied to the front surface 2 of the membrane, the total charge removalis optimised to 0.19 C/mm² in order to provide a high quality of surfacefinish and to minimise the incidence of over-etching.

Advantageously and in order to further reduce the risk of subsequentlyover-etching of the nozzle throat portion 12 during the post-processcleaning and electro-polishing, the length of the throat portion 12 maybe increased through appropriate adjustments to the drilling process.This is indicated by the difference between the nozzles shown in FIGS.1(c) and 1(d). This ensures that for a certain spread in the geometry ofthe throat portion, resulting from the tolerance associated with thehigh-speed laser drilling process, more nozzles will have sufficientlength of throat portion 12 to prevent over etching during theelectro-polishing process.

FIGS. 6 and 7 provide a graphical representation of the populationdistribution of nozzles 10 on two sample membranes 1 for which thenumbers of pulses used to drill through the throat portion 12 isindicated on the x-ordinate. Each membrane 1 was drilled using differenttarget numbers of pulses, N_(t) within the N_(error) algorithm, fordrilling the throat portion 12 of the nozzle in the laser-drillingcontroller. This distribution data is used to indicate the quality ofthe nozzle-geometry within each membrane. If this distribution is wide,then a wide distribution in geometry is anticipated in the throatportion 12 on that membrane 1. Similarly if this distribution is narrow,then a narrow distribution in this geometry is anticipated. In FIGS. 6and 7, the target number of pulses was set to 11 and 15 respectively,for the number of pulses to drill the throat portion 12. For the samemembranes, the number of pulses set to drill the diverging portions 11were set to 20 and 16 respectively, thus maintaining the 31 pulsesetting to drill through the overall thickness of the membrane in bothcases.

It was found that the membrane 1 from which the data for FIG. 6 wastaken contained nozzles with roughly 2:1 ratio of length for thediverging and throat portions respectively, in proportion with thenumber of laser pulses required to drill these portions being 20:11,respectively. It was also found that the membrane from which the datafor FIG. 7 was taken contained nozzles with roughly a 1:1 ratio oflength for the diverging and throat portions respectively, in proportionwith the numbers of laser pulses required to drill these portions being16:15. The variation between the number of pulses required to drill thenozzles 10 on each membrane 1 results in a variation in the length ofthe throat portions 12 of these nozzles. In FIG. 6 this variationresults in a finite population of nozzles which have been drilled withbetween two and five pulses. While this is less than half the targetnumber of pulses set to drill the throat portion 12, these nozzles willbe almost indistinguishable through optical transmission measurements ofminimum cross-sectional area or diameter. However, it is clear that thethroat portion 12 of these nozzles is substantially shorter than thetarget length, and may be as short as 10% of the overall membranethickness. During post process cleaning of such a membrane, over-etchingof the diverging portion 11 of these nozzles may result in a substantialincrease in the diameter throat portion 12. In this case the minimumcross-sectional area of the nozzle will be relatively uncontrolled, andmay easily expand towards that of the diverging portion. For examplenozzles with a drilled throat portion 12 diameter of ø3.0 μm diametermay easily increase this diameter to over ø15.0 μm.

The membrane 1 used to generate the data in FIG. 7 possesses no nozzleswhich have throat portions drilled in the range two to five pulses. Infact the minimum number of pulses used to drill any throat portion inthis membrane was 10 pulses, thereby the minimum throat length is nearly40% of the membrane thickness. This increase in the throat thickness hasresulted in more than a ten-fold decrease in the population ofover-etched nozzles on these membranes after electro-polishing.

1. A method of forming perforate membrane for use in a liquid transportdevice, by applying laser energy selectively to a first surface of themembrane in the form of a pulsed, focused beam to form a plurality ofnozzles each having a throat portion opening at one end through theopposite surface of the perforate membrane and a smoothly curvedoutwardly diverging portion extending from the other end of the throatportion to the first surface of the perforate membrane, comprising thesteps of: thereafter electro-polishing the first surface of the membraneand the surface of the diverging portion of the nozzles to removesurface imperfections, and controlling the electro-polishing so as toremove material from the surface of the diverging portion of the nozzlesto a depth less than the length of the throat portion.
 2. A methodaccording to claim 1, further comprising electro-polishing the oppositesurface of the membrane and the surface of the throat portion of thenozzles to remove surface imperfections.
 3. A method according to claim1, wherein the laser energy is applied selectively to the first surfaceof the membrane in the form of a pulsed, focused beam.
 4. A methodaccording to claim 3, wherein the laser energy is applied in two stepsto form each nozzle, between which steps the distance between the laserfocus and the first surface of the membrane and/or the pulse energy ofthe laser beam is adjusted.
 5. A method according to claim 3, wherein,for forming each nozzle, a focused laser beam is applied to the firstsurface of the membrane positioned at a distance from the laser focus,to remove material of the membrane in order to form the divergingportion, and thereafter the distance between the laser focus and thefirst surface is decreased and/or the pulse energy of the laser beam isreduced, in order to form the throat portion extending from thediverging portion to the opposite surface of the membrane.
 6. A methodaccording to claim 5, wherein the distance between the laser focus andthe first surface of the membrane and/or pulse energy of the laser beamis adjusted during the step of forming the diverging portion.
 7. Amethod according to claim 5 or claim 6, wherein the distance between thelaser focus and the first surface of the membrane and/or pulse energy ofthe laser beam is adjusted during the step of forming the throatportion.
 8. A method according to claim 1, wherein the position of thelaser focus relative the first surface of the membrane and/or therelative laser pulse energy is controlled by comparison of the number ofthe pulses required respectively to form the diverging portion or throatportion with a predetermined respective number.
 9. A perforate membraneformed by a process according to claim
 1. 10. A membrane according toclaim 9, wherein the outwardly diverging portion of each nozzle isconcave.
 11. A membrane according to claim 10, wherein the concaveportion is part spherical.
 12. A membrane according to claim 9, whereinthe throat portion is substantially cylindrical.
 13. A membraneaccording to claim 9, wherein the throat portion is divergent towardsthe one surface of the membrane.
 14. A fluid transport device includinga perforate membrane according to claim
 9. 15. An aerosol generatorincorporating a membrane according claim
 9. 16. A pump incorporating amembrane according to claim
 9. 17. A filter incorporating a membraneaccording to claim 9.